Well, my internet search history is looking pretty weird now, but I wanted to check this one out, since I was under the impression that adopted siblings are generally treated like full siblings, including for incest laws. It looks like adopted siblings marriages are now allowed in most U.S. jurisdictions, but not everywhere. Here's what one website had to say about it:
Statutes prohibiting adopted siblings from marrying have been challenged and held to be unconstitutional. Most courts, but not all, allow marriage by persons related only by adoption.
Example: An adopted brother and sister want to marry each another. Their parents had married when they were ages 18 and 13, respectively. They are denied a marriage license because of the prohibition against brother/sister marriage. In this case, the brother and sister argued that the statute prohibiting their marriage was unconstitutional because the statute did not have a rational relationship to a legitimate state interest. In particular, it violated the Equal Protection Clause of the U.S. Constitution. Furthermore, since they were not related by blood, there was no problem of producing genetically defective children. The court agreed, and they won the right to marry. See, e.g., Israel v. Allen, 195 Colo. 263, 577 P.2d 762 (1978).
A marriage between people related by affinity or by marriage, used to be widely prohibited and still is today, with respect to step-relationships. It is justified not on a genetic basis, but on social grounds - maintaining a harmonious family. Courts do not like to authorize and encourage the marriage of adopted brothers and sisters because it would undermine the fabric of family life and would go against the purposes which the adoption process was intended to serve. See, e.g., Marriage of Mew and MLB, 4 Pa. D & C.3d 51 (1977).
In the absence of a statute, when a marriage ends by death, divorce or annulment, most courts terminate all affinity relationships that were produced by the marriage.
